In reference to geothermal wells in water-sensitive strata in Xuchang city， Henan province， and based on the analysis of the composition， hydration mechanism and drilling mud performance requirements of water-sensitive strata， drilling mud with small filtration， low density， good rheological property and strong inhibition was specified for the water sensitive formation. Based on the literature and field experience， the components and their concentration for mud preparation were preliminarily selected， mud performance was determined by laboratory test， and component concentration was optimized by orthogonal test range analysis with the final mud make-up optimized. With field verification and adjustment， the mud formula suitable for field use was finally determined. field test has proven that it met the drilling requirements of water sensitive strata with good effect.